RegExp constructor ignoring given flags as second argument leading to TS errors about typescript HOT 7 CLOSED

"This is a crash" That means the compiler itself crashed. If the errors you're seeing are in your own code, or in code being compiled, it's not a crash. Only if the errors are a stack trace of the compiler code itself is it a crash.

This doesn't look like a bug to me. The problem is in /(\p{L}{1})\p{L}+/ itself. For that to be a valid regex it needs to have the u (or v) flag, as those unicode property value expressions are only supported in Unicode-aware mode. Certainly the equivalent code:

const re = /(\p{L}{1})\p{L}+/; // error
// later
let rgx = new RegExp(re, "gu");

should be an error, right? Because the thing you assigned to re is not valid. The fact that you later fix it doesn't change that. So are you saying there should be a carve-out for when invalid regexes are created inline? I'd think it would be better never to write /(\p{L}{1})\p{L}+/ in the first place, and just write /(\p{L}{1})\p{L}+/u, at least, which makes it valid. Once you do that the error goes away:

new RegExp(/(\p{L}{1})\p{L}+/u, "gu"); // okay

Of course then there doesn't seem to be much benefit over writing /(\p{L}{1})\p{L}+/gu in the first place. Indeed, making a regex only to throw it away while building another regex seems weird to me. If I saw someone write const x = [...[1, 2, 3], 4] instead of const x = [1, 2, 3, 4], I'd be very puzzled. Seems like the same thing here.

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@anthonyLock I think the RegExp constructor is primarily for creating dynamic regexes, whose content is not known at compile-time. Situations like this:

function escape(s: string) {
	return s.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
}
const searchValue: string = getSearchValueFromInputField();
const escaped = escape(searchValue);
const testRegex = new RegExp(escaped, "i");

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if you change the target to es5 then you get the error This regular expression flag is only available when targeting 'es6' or later.

That is correct. These features are not available in es5. If this works at runtime:

new RegExp(/(\p{L}{1})\p{L}+/, "gu").test('AB') === true

then the JS engine you are targeting is not es5.

and will cause confusion for others

I disagree. The regex /(\p{L}{1})\p{L}+/, without the u flag, is extremely confusing:

/(\p{L}{1})\p{L}+/.test('p{L}p{L}}}}}}') === true

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Why not just write this as /(\p{L}{1})\p{L}+/gu?

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@nmain if you change the target to es5 then you get the error This regular expression flag is only available when targeting 'es6' or later. Its not effecting me right now as I have changed it for the string approach let rgx = new RegExp('(\\p{L}{1})\\p{L}+', 'gu') but its a bug regardless, and will cause confusion for others.

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Agree with prior commentors.

There's also the option of new RegExp("(\\p{L}{1})\\p{L}+", "gu")

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@jcalz @nmain

Thanks for the information. I have closed the issue, if an admin wants to delete the issue feel free. I didn't realise that regex in that form was not supported in es5. Somewhere in my project between the setup and use of babel,nextjs etc... there is a configuration issue.

Quick question just to improve my own knowledge, please could you explain the use case of using the second variable in the constructor for RegExp and when would you would you want to use it instead of adding it into the first input?

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